MAXI 7 Years Highlights
Tatehiro Mihara,1 and the MAXI team
1 MAXI team, RIKEN, Wako, Saitama, Japan

E-mail (TM): tmihara@riken.jp
ABSTRACT

Since MAXI started observation on August 15, 2009, MAXI is monitoring the X-ray sky for morethan 7 years. The light curves of about 350 sources are updated in every 4 hours or 1 day and open
through MAXI homepage (http://maxi.riken.jp). Alerts from MAXI on new X-ray transients are sent
to subscribers in the MAXI alert mailing lists at 12 seconds after the emerge as the earliest case.
Many of them were followed up by the Swift/XRT and the nature of the source were revealed.
Seventeen new sources were found by MAXI. Six of them were new black holes (BH). Soft X-ray flash
at an ignition of a nova was detected for the first time in 2011 November 11. Tidal disruption events
have formed a new category of transients after the discovery of Swift J1644+57. MAXI was usefulthat it had motnitored the source activitity before the event.
MAXI catalogs of both GSC and SSC were issued listing new members of X-ray emitters in the 21st
century. Fluxes of X-ray binaries are monitored continuously recording histories of activities of theneutron stars (NS) and BH. Monitoring data is used for detailed observations with Suzaku, Chandra
and XMM-Newton. Data with uniform quality are used for power-spectrum-density analysis in AGNs
and study of accretion disk instabilities in NS-LMXB and BH binaries. Normal and giant outbursts of
Be X-ray binaries are monitored enabling a detailed discussion on the Be stellar disk together with
optical emission-line profiles. MAXI detected large flares from blazars as well as those from active
stars, and one-hour lasting superbursts from NS-LMXB. Gamma-ray bursts have been detected
steadily, whose number reached 85. Still, we had 7 MAXI unidentified short soft transients (MUSST).
In 2015 LIGO started operation and found the gravitational wave sources. NICER, a new X-ray
telescope on the ISS, will be mounted in early 2017. Together with these new instruments, MAXI will
open new windows to understand the universe further.
KEY WORDS: MAXI, All-sky monitor, Transients, Time-domain astronomy
1.

MAXI instruments

On 2009 July 16, MAXI was launched by the
space shuttle Endevour (Matsuoka et al. 2009). A
Japanese astronaut Koichi Wakata moved therobotic arm to mount MAXI to JEM-EF port #1(Figure 1). MAXI/GSC (Mihara et al. 2011, Sugizaki et al. 2011) has two field of views (FOV),
Horizontal and Zenithal, each with 160o×3o FOV.
Since the operation of GSC is limitted to the low
background region around the equator, the fraction of the working time is ~0.4 (Sugizaki et al.
2017). Even though, MAXI/GSC can scan almost
the whole (90%) sky in one orbit (92 minutes)
thanks to the two FOVs. Dwell time of one MAXI
scan is ~60 s, and the 5σsensitivities are ~130mCrab / 1 scan, ~20 mCrab / 1 day, ~4 mCrab / 1
month, and ~1 mCrab / 1 year (Negoro et al.
2016).

Figure 1. Real MAXI on ISS. Photo taken from thedocked space shuttle (©NASA). Right-hand side is
the moving direction of ISS. MAXI has two FOVs.

Figure 2. First light of MAXI/GSC on 2009 August
15. This image was press-released by JAXA and
Riken. Two scan poles with 10o radius move on δ=±23o lines with a presession period of the ISS orbit(70 days).

2.

Press releases, papers and awards

There are 13 press-releases from MAXI starting with the first light of MAXI/GSC (Figure 2).
The first X-ray nova for MAXI, XTE J1752-223,
was press-released from the Astronomical Societyof Japan in 2010 September as “slow-eating BH”(Nakahira et al. 2010). Press-releases of scientificresults followed as a BH nova MAXI J1659-152(Yamaoka et al. 2012), a slow X-ray pulsar MAXI
J1409-619 (Yamaoka et al. 2011), a tidal disruption event Swift J1644+57 (Burrows et al. 2011),
Cygnus super bubble (Kimura et al. 2013), a
massive nova ignition MAXI J0158-744 (Morii etal. 2013), a nearby monster GRB 130427A (Maseli
et al. 2014), and a optical quick (~2 minutes)
followup of V404 Cyg (Kimura et al. 2016).
Refereed papers using MAXI data reached 192(as of 2017 February). Those by MAXI team are
62. Praising ~100 citations, the PASJ Excellent
Paper Award 2013 was given to Matsuoka et al.
(2009) by Astronomical Society of Japan (Figure3). We had three MAXI conferences in 2008, 2010,
and 2016. Admiring the MAXI leading of thecurrent time-domain astronomy and multimessenger astronomy, ISS R&D Conference 2016
Innovation Award in Earth and Space Science
was given to the MAXI by CASIS (Center for the
Advancement of Science In Space), American
Astronautical Society, and NASA.

Figure 3. PASJ Excellent Paper Award 2013 to the
MAXI misison paper, Matsuoka et al. (2009).

3.

Scientific results

3.1. MAXI all-sky map and catalog
The GSC all-sky map was presented in Mihara(2014). The SSC all-sky map was published by
Kimura et al. (2013). The 37 months MAXI/GSC
catalog in high galactic region was by Hiroi et al.
(2013). In there, 500 sources were detected with ahigher significance than 7σ. The detection limit
was ~0.4 mCrab. The log N - log S relation isconsis- tent with HEAO-1’s result in fluxes >10-11
erg cm-2 s-1. However, the 40% of object members
had changed since 1970’s. Thus MAXI has
created a catalog in the early 21st century.
3.2. MAXI transients
MAXI discovered 17 new sources in addition to
85 gamma-ray bursts (Negoro et al. 2017). The
breakdown is 1 white dwarf binary, 6 NS, 6 BH,
and 1 unknown. The completely new kind of
phenomenon was massive nova ignition MAXI
J0158-744 (Morii et al. 2013). On 2011 November
11, a bright (~ 1 Crab) but soft ( < 5 keV) X-ray
transient appeared near SMC. Fortunately both
H and Z-SSC detected it. It was not detected in
the next GSC scan (Figure 4). About 11 hours
later Swift/XRT detected a vary faint (10-3 of
MAXI’s flux) source. It was identified to a Be star(B1-2 IIIe) at SMC distance ( = 60 kpc). Then theflux was 1040 erg s-1, which exceeds the Eddington limit for 1 Mo by a factor of 100. The GSC
spectrum can be fitted by any of power law, black
body (BB), and thermal brems. If BB was applied,
the temperature was 450 eV and the radius was
103 km. The Swift spectra can be fitted by BB
with 60 → 110 eV and area of 104 → 102 km.
From the SSC-Z observation, an ionized Ne line
was detected. Theoretical explanation followed(Ohtani et al. 2014). The size and temperature
are typical to a white dwarf (WD) and the light
curve was similar to that of a classical nova. The
difference was the faster time-scale and thehigher temperarure. The MAXI part was interpreted as an ignition phase of a nova, which wasobserved for the first time in the history of nova.
Also, it was soft X-ray against the expectation of
UV emission. The Swift part corresponds to thesuper soft source (SSS) phase. The fast time-scaleand high temperature can be explained by a very
massive (nearly Chandra- sekhar limit of) WD.
Thus the system would be a vary rare one
composed of a massive O-Ne WD and a Be star.

3.4. Monitoring X-ray binaries (XRB)
Activities of NS- or BH- XRB are monitored
once in 92 minutes and made public at MAXI HP1
every 4 hours or every day. Periodical and nonperiodical outbursts from BeXRB were presented
by e.g. Mihara et al. (2014) and Nakajima et al.
(2017). The MAXI data are compiled in BeXRB
activity web page2 by ESA/ESAC.
The non-periodic outbursts of NS low-mass
X-ray binaries (LMXB) are studied by Asai et al.
(2012), where repetition of outbursts in several
hundred days were clearly shown. Sudden decrease in the last part of an outburst was related
to the propeller effect, from which magnetic fieldsof the NS was estimated (Asai et al. 2013). A
small outburst without spectral state transition
was named “mini outburst” in Asai et al. (2015).
It was an observational evidence of purr-type
disk instability predicted theoretically. Outburstsof BH are also watched by seveal researchers(Figure 5).
A long-term monitoring of an XRB is also useful for detailed observations. Chandra observation by Heinz et al. (2015) showed giant X-ray
light echoes around Cir X-1. This “Lord of theRings” was understood using the light curves of
MAXI in hundreds of days.

Figure 4. X-ray light curve of MAXI J0158-744.
After trigger by GSC-H, SSC-H and SSC-Z detectedsuccessively (Morii et al. 2013).

3.3. Huge stellar flares
MAXI detected huge stellar flares unbiasly.
Those are from nearby (~100pc) active stars: dMestars and RS CVn (and Algol) binaries. The peakluminosities are 1034-1036 erg s-1 and 1036-1039 erg
s-1, respectively. These are 103-107 times larger
than the largest solar flares, and even 1-104
times larger than the super flares of G-type stars(Tsuboi et al. 2016, Tsuboi et al. 2017).

Figure 5. X-ray light curves of BH binaries. Taken
from MAXI HP 3 . Hardness curves, intensityhardness plots and spectra are also available.

http://maxi.riken.jp/
http://integral.esac.esa.int/bexrbmonitor/
3 http://maxi.riken.jp/mxondem/outburst/BH/
1
2

The pulsation was mointored with MAXI from MAXI detected 12 superbursts including intersome X-ray binary pulsars. The 7.67s pulsation of mediate bursts, among 28 ever detected (Serino
4U 1626-67 is detected in each 60-day. Since et al. 2016).
7.67s is short enough compared to the ~60 s scan
Taking the advantage of MAXI’s continuous
time, we can get rid of uncertainty by variablesampling, a long term power spectrum density
background and source-flux in the folding process.
(PSD) was studied. In Cyg X-1, long-term PSD
For the intensity of 30 mCrab, we need 60-day
down to 10-7 Hz was obtained by Sugimoto et al
integration to detect a pulsation. In the 7 years of(2016) for both soft state and hard state.
MAXI observation, 4U 1626-67 increased the
Two-layered accretion disk was discussed based
luminosity by 20%. The spin up rate is related toon it. They needed to correct observational gaps
a flux. In the historical view, 4U 1626-67 repeats
and aliases arising from the sparse-sampling by
spin-up and down. Together with historical resnap-shot observations. Premitive PSDs of manysults, the P-dot and flux relation is well explained
sources without corrections are publicly available
by the Ghosh and Lamb (1979) theory (Figure6).
3.6 活動銀河核from MAXI HP4.
Their n(ω) function with the fastness parameter
高い X 線検出感度と大きな視野をあわせ持つ MAXI は、活動銀河核（AGN）の活動の監視に大きな威力
ω can handle both spin-up and down. The
surを発揮する。実際、
MAXI は、活動銀河中心核の一種であるブレーザー天体の長期 X 線監視観測で、観測
variabilityの初期段階から重要な成果
たとえばIsobe et al., 2010a) をあげてきた。
face magnetic filed is known from the cyclotron 3.5. (AGN
AGNの中には、ジェット呼ばれる超高速のプラズマ流を噴出する天体がある。その中でも、ジェットがfeature. Then we can determine the mass andSome AGN show rapid variability. The Xmost
我々の視線方向を向いた天体がブレーザーである。ブレーザーの大きな特徴に、電波から線, γ 線に至る
radius of the NS if we know the distancevariable one is the BL Lac objects. Figure 7
広帯域の非熱的放射と、その強度の激しい変動があげられる。その時間変動の様子を詳細に解析すること
accurately enough (Takagi et al. 2016). で、磁場や放射領域の大きさなどジェットの重要な物理量を推量することが出来る。
shows 3-day
flares of Mrk 421. X-ray flux

図 59 に、2009 年 8 月の通常観測開始からの 3 年間に蓄積した、ブレーザー天体 Markaryan 421 (以下changed by more than one-order-of-magnitude inSuperburst is a long X-ray burst in NS-LMXB
Mrk 421) の X 線光度曲線である。この天体は、全天でもっとも X 線フラックスが大きいブレーザーで、
several days. Long-termMrkPSD
of Mrk 421 has a
MAXI
421 が、非常に活発に活動している状態と静lasting ~3000s. Normal X-ray burst is a H
and
He
Mrk
421で最も精度のよい観測データを取得できている。
のX 線フレア後の速報を元に JVN による素早いモニター観測を実施し、新しいジェットの blob が-5 Hz. In lower MAXI
穏な状態を繰り替えしている様子が、はっきりと分かる。特には、Mrk 421が一日程度のタイムス
break
at
10
frequency
than
that,
burning, while the superburst contains
a C
噴出した兆候を発見することに成功した(Niinuma et al., 2012) 。これは、ジェットの噴出と強い
X 線フレ
ケールで突発的に増光する現象（フレアと呼ぶ）をAstronomer’s
Telegramを通じていち早く-8 Hz
the PSD extended in3a回検出し、power-law
down
to 10
アの発生を関連付ける、重要な成果と考えられる。
burning in addition to H and He. Sincethe
全世界に速報することに成功した (Isobe et al., 2010b,c; Hiroi et al., 2011) 。
さらに、我々は(Isobe
3 年間という長期に渡って取得された
MAXI の光度曲線を解析することで、その変動のet al. 2014).
intensity and duration match MAXI scanning,

パワースペクトル密度をかつてない長い周波数範囲にわたって決定することに成功した（図 60）。過去の観測結果によれば、t ∼1 day 以下のタイムスケールにおいて、Mrk 421 のパワースペクトル密度は、指数≈2.1 のべき関数で近似できることが知られていた (Kataoka et al., 2001) 。今回、MAXI により初めて長いタイムスケールでの変動特性を測定した結果、t ∼10 day 以上でのパワースペクトル密度のべきは ≈1.6 であり、より高周波数での傾きより有意にフラットになっていることが明らかになった。このフラットなべきは、強いジェットをもたない通常の AGN（セイファート銀河）から観測される値とほぼ一致している。こ
の事実は、ブレーザー天体における長いタイムスケールの変動は、ジェット内部での現象でなく、ジェットを形成する降着円盤の変動によって作られていることを示唆している。この結果は、「ブラックホールからの相対論的ジェットの形成機構」という重要課題に対して、極めて重要な示唆を与えるものである。

図 59: MAXI GSC が 2009 年 9 月から 2012 年 10 月の間に観測したブレーザー天体 Mrk 421 の 3–10 keV での X 線光度曲線。菱形（赤）が観測点で、点線（黒）は非観測時期の線形内挿を表す。MAXI が Astronomer’s
Telegram に速報した X 線フレア (Isobe et al., 2010b,c; Hiroi et al., 2011) を、 図上の矢印で示した。Isobeet al. (2014) より抜粋。

フレアの際の X 線光度曲線を詳細に調査することで、Mrk 421 のジェットの物理量を推定することにも成功した。例えば、フレアのピークの減衰時間は、ジェット中の高エネルギー粒子がシンクロトロン放射でそのエネルギーを失うタイムスケールで決まると考えられるため、減衰時間からジェットの磁場を測定出来る。MAXI
Figure 6. Flux and pulse period derivativeof 4Uが測定した X 線光度曲線によると、1 月 1 日のフレアのピークの減衰時間は約 25000 秒であり、これは磁場の強度 B ∼ 0.045(δ/10)−1/3 G に対応する（δ はジェットのドップラー因子）。これは、これまで
1626-67 with MAXI and previous results. Theの研究で報告されてきた磁場強度とも矛盾がなかった (B = 0.036 – 0.44 G; Kino et al., 2002) 。いっぽう、
best-fit model is by Ghosh and Lamb (1979).
In年this2010
2 月 17 日のフレアのピークは減衰するのに約 1 日半もったことから、このフレアを起こした領域のplot we added a flux uncertainty of ±5.5% so
asBthe磁場（∼ 0.015(δ/10)−1/3 G）はこれまでの観測と比べて有意に弱かったことが示唆された。この結果をもとに、巨大なフレアは磁場の弱いところで起きる、あるいは巨大なフレアはいくつかのフレアの重ね合わχν2 to be 1.0. If we assume the dis- tance D=10
kpc,
せで起こる、という新たな可能性を提示することが出来た (Isobe et al., 2010a) 。
we can obtain the mass M = 1.74±0.05 図M60:o,and およびMAXIASCA 7.
で得られた、
Mrk 421lightのペリオドグラム（周波数がかかっている）。ポアソン
Figuretop : MAXI
curve of Mrk 421.
MAXI がフレアを速報したことにより、ブレーザーの他波長連携観測の可能性が大きく広がった。例えば、レベルは差引済。赤四角はによる結果を、青四角は
ASCA
による結果
al., 2001) を示
radius R = 13.5±0.1 km of the NS very accurately.
bottom: long-term
PSD of Mrk
421(Isobe (Kataoka
al. et VERITA
2010 年 2 月 17 日のMrk 421MAXIのX 線フレアに同期して、非常に強い超高エネルギー
γet
線フレアが
す。白三角は、のデータ点から得られたパワースペクトル密度のベストフィ
ットモデル（指数 1.6 の望遠鏡などにより報告されている(Ong, 2010; Bartoli et al., 2011) 。また国内では、「かなた」望遠鏡など
The dashed and the dotted lines correspondto MAXI
2014)べき関数）より予想されるペリオドグラム。黒三角は、
ASCA で決めた高周波数側のべき指数（2.14）を、の 可視光観測や大学 VLBI 連携 (JVN) などの電波観測との協力体制も構築しつつある。実際、
2 月 17 日のchanges of the radius by 1 km and the mass by 0.3
そのまま低周波数側に延長した場合に予想されるペリオドグラム。低周波数側と高周波数側で、パワースペ
4 http://maxi.riken.jp/PSD
Mo, respectively. (Takagi et al. 2016).
クトル密度のべきが異なることが分かる。
Isobe et al. (2014) より抜粋。
55

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3.6. Gravitational wave & unknown transients
Last year the histrical detection of the first gravitational wave (GW) was reported. GW 150914
occurred at 2015 September 14 9:50:45 UT.
Unbiased survey of MAXI is suitable for watchingan X-ray counter part of a GW. Unfortunately
MAXI was not operating at the time of the GW
detection because it was in a high backgroundregion. In 4 minutes later, MAXI resumed theoperation and 3 σ upper limit (2-20 keV) was
obtained as 0.1 c s-1 cm-2 ~ 30 mCrab ~ 10-9 erg s-1
cm-2 (Kawai et al. 2017). MAXI also put upper
limits in flux just before, long before and longafter the event. For the second one GW 151226
MAXI was operating at the occurrence, but justan upper limit was obtained (Serino et al. 2017).
In the seven years of unbiased operation, MAXI
has accumulated unknown soft short transients.
In spite of the Swift/XRT followups, no counterpart was detected for the seven sour- ces (Table 1).
We named it MAXI unidentified short softtransient (MUSST). In Figure 8 MAXI J1501-026
was shown as an example. It is soft : detected
only with MAXI in the soft X-ray band (2-10 keV)and not detected by Swift/BAT nor GRB detectors.
It is a short transient not detected in the next
scan of MAXI, and unidentified because ofundetection by the Swift/XRT follow-up.

Table 1. Seven MUSST sources
Name
Flux
MAXI / GRB
Date(Crab) Atel/GCN
M J1501-026
2015-8-26
0.44
A #7954
G 150428C
2015-4-28
0.16
G#17772
M J1540-158
2015-3-11
0.1
G#17568
G 140814A
2014-8-14
0.23
G#16686
M J0545+043 2014-4-12
0.2
A #6066
G 130407A
2013-4-07
0.17
G#14359
M J1631-639
2011-4-29
0.12
A #3316
We reported some sources with GRB names. But
they would not be GRB because they were soft anddid not accompany X-ray afterglows.

c/s/cm2

Figure 8. MAXI J1501-026 as an example of
MUSST. left : MAXI image at detection. It is very
bright. middle: light curves of MAXI scan in threeenergy bands. There was no flux in the hard band
(10-20 keV). right : X-ray image of Swift/XRT
followup. No source was detected in the MAXI error
region.

4.

MAXI future

MAXI’s operation was adopted untill 2018
March. In terms of the Japanese contribution toISS, JAXA continues to join the ISS program till
2024. They call it “op-3”. We MAXI team will
apply for a further 2-3 years of extension of MAXI
operation. New science points in this extension
will be a search for an X-ray counter part of GW
and revealing the MUSST. In general words,
search for the discovery space in the time-domain
astronomy (Figure 9). Here cooperation with new
instruments is a key. The JEM mission CALET isoperating since 2015 (Nakahira et al. 2017).
CALET/GBM observes GRB simultane- ouslywith MAXI. Advanced LIGO is operating since2014 to detect a GW. Currently O2 is going on.
But the most advance will be coorporation with
NICER.
NICER is a new ISS instrument which will belaunched in 2017 April. It will be attached to
ELC3 truss in the NASA part. NICER is an arrayof pointing X-ray telescopes without imaging.
Besides the main purpose of tracing the pulse
light curve of a single NS, it will be capable of aquick follow-up of a new MAXI transient. We
MAXI team and NICER team proposed the
OHMAN (On-orbit Hookup of MAXI And NICER)
project to connect the two on orbit for the
real-time follow-ups. The point is “Watch the
X-ray transient in X-ray while it is still bright”.

observations of ground observatories andsatellites in orbit. New phenomena (as an
ignition of a nova) and six black holes werediscovered. MAXI has opened a new era oftime-domain astronomy and of multi-messenger
astronomy with the highly-sensitive X-ray all-sky
monitor and the real-time alert. Together withnew instruments such as gravitational wave
detectors and the X-ray detector NICER on ISS,MAXI will be on the cutting edge of the X-ray
astronomy.
References
Figure 9. Rapidness of follow-up in X-ray andsensitivity. Discovery space in the time-domain
astronomy is shown. Some kinds of quick decaying
objects can be detected with the on-orbit MAXI –NICER connection. Here we assume
MAXI: When the explosion started in MAXI FOV,NICER: MAXI alert on-orbit and via-ground,
Swift: MAXI alert through GCN. Exposure of 1ks.

Besides the cooperation with new instruments,
continuing MAXI operation can make follows.
・ Coordinated observation of transients in
multi-wavelength in Japan and internationally.
New black hole binaries : 1-2 / yr
Huge stellar flares
Low-mass X-ray binary and super bursts
Gamma-ray bursts
Tidal disruption events
X-ray binary transients・3rd catalog in low galactic region (|b|<10°)
Hori et al. (2017）and more sources.
More X-ray photon statistics. Sensitivity
reaches confusion limit (~0.5 mCrab).
The
number of sources reaches ~1000.
With Light curves and variability index.
i.e. information in time-dimension, too.
・ Permanent MAXI archive at ISAS (Ebisawa
2017)
5.

Summary

Continuous monitoring of MAXI on ISS has
been providing basic information on variability of
X-ray sources, which is distributed freely to theworld. A real-time alert triggers many follow-up

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